The present invention relates generally to dynamic braking of D.C. motors and, more particularly, to a motor control system for dynamically braking a motor in a barrier movement operator.
Most generally available barrier operators when stopped in mid-travel come to a halt very abruptly. This is most easily observable, for example, when a garage door opener is moving a door from an opened to closed position and is reversed or stopped, such as when infrared sensors detect an obstruction or when a control button for stopping or reversing the barrier operator is depressed. The garage door, which can weigh upwards of 250 pounds, can be seen to shudder to a stop. During a reversal operation, the barrier jitters and bounces, causing the chain track and the entire housing of the barrier operator to shake in its moorings. The sheer weigh and velocity of the door, combined with having to reverse or stop suddenly, puts a strain on the entire system.
The above phenomenon is caused by the manner in which the barrier movement operator is controlled. Common DC control arrangements consist of a relay or other switching apparatus to control the applied DC potential, and some form of power regulator to connect portions of power from a DC supply to the motor. The variable power connection may consist of something as simple as a rheostat or something more complex such as a semiconductor switching arrangement. Although the power couplers may vary in sophistication, the system is basically a source of DC power coupled by a regulator to the motor or other power using device.
Referring to FIG. 1, a typical barrier movement operator is illustrated. As shown, the operator includes an AC line source 10 for providing power to the system. The AC voltage from the AC line source 10 is filtered by a filter 12 then converted into DC voltage by a rectifier 14 for use by a load, such as a DC motor 16. The barrier operator is slowed to a stop at the limit by a controller 18 that pulses transistor Q120. Transistor Q120, as used herein is a N-channel field effect transistor (FET). However, other types of transistors may also be used with appropriate circuit modifications performed by those skilled in the art. Relay122 and Relay224 control the direction of the motor. Pulsing transistor Q120 causes the voltage being applied to the motor to be turned off and on and has the effect of reducing the supply voltage. Once supply voltage is reduced or eliminated, the friction in the system stops the door travel. A particular disadvantage of such a system is that the barrier operator is stopped at a desired position only when the rate of frictional stopping is greater than the desired electrical stopping.
Turning now to FIG. 2, the operator is shown wherein the rate of frictional stopping is greater than the desired electrical stopping. The line voltage 30 supplied by the AC line source 10 (FIG. 1) is passed through the full wave rectifier 14 to produce a fully rectified signal 32 for use by the motor 16. When the operator 110 is commanded to stop, the controller 18 pulses the power 34 being supplied to the motor 16, thereby reducing the voltage to the motor and causing the motor to stop. The frictional rate of stopping is adequate because the voltage 36 across the motor 16, when the motor is acting as a generator, is lower than the peak voltage 38 of the power supplied from the rectifier 14. Thus, the door comes to a stop in the desired position.
In contrast, referring to FIG. 3, it can be seen that problems arise in those instances where the inertia of the barrier operator is so high that the rate of frictional stopping is slower than intended. As above, the rectified voltage 32 is supplied to the motor 16. When the operator 110 receives a stop command, the controller 18 provides a pulsed voltage 34 to the motor 16, thereby having the effect of reducing the supply voltage. However, in this instance, the frictional rate is inadequate because the voltage 40 generated by the motor 16, which is now acting as a generator, is greater than the peak voltage 42 of the power supplied by the rectifier 14. Thus, the barrier is unable to be stopped at the desired rate and the operator 110 cannot be stopped simply by pulsing the transistors. As such, in those instances where the barrier operator is unable to slow the movement of the barrier, stopping the operator in a panic situation must be accomplished by shorting the motor, or allowing the barrier to strike a physical limit, resulting in a very sudden stop. Unfortunately, such abrupt stops create high forces acting on the door and the operator, as described above, which result in undue wear and tear on the operator and high stresses on the barrier.
What is needed, therefore, is an operator controller for softening the deceleration of the operator and the door.